A CT scanner generally includes an x-ray tube that emits ionizing radiation that traverses an examination region and a portion of an object or subject therein and illuminates a detector array disposed across the examination region, opposite the x-ray tube. The detector produces projection data indicative of the detected radiation. The data can be reconstructed to generate volumetric image data indicative of the portion of the object or subject. With spectral CT, the projection data includes signals which are acquired concurrently and that correspond to different photon energy ranges. There are several approaches for performing spectral CT. For example, the CT scanner may include two or more sources, at least one source configured to switch between at least two different kVps, and/or a detector array with energy-resolving detectors.
With spectral CT, two acquired signals can be used to determine the photoelectric and Compton contributions of each signal and identify an unknown material by its value of photoelectric and Compton contribution. Generally, because any two linearly independent sums of two basis functions span the entire attenuation coefficient space, any material can be represented by a linear combination of two basis materials. This works especially well in materials, such as iodine, that have a k-edge energy close to the mean value of a diagnostic energy range. Furthermore, the additional spectral information improves the quantitative information that can be determined about the scanned object and its material composition. The basis material also allows for generating a monochromatic image, a material cancellation image, an effective atomic number image, and electron density image.
Again, CT scanners emit ionizing radiation. Unfortunately, ionizing radiation may damage or kill cells and/or increase the risk of cancer. The literature has indicated that dose levels from CT typically exceed those from conventional radiography and fluoroscopy. However, the radiation dose for a particular imaging procedure cannot just be lowered as a lower dose leads to increased image noise and thus blurrier or un-sharp image. Moreover, spectral CT images are already inherently noisier than conventional non-spectral images. For example, in a dual energy study, each image is based on roughly half of the radiation dose of a corresponding non-spectral conventional scan. Furthermore, the estimate of the material decomposition is based on projections between two vectors with a narrow angle there between. The combination of these two factors, i.e., large noise and narrow angle, amplifies significantly the noise in the estimated material decomposition.
Contrast enhanced CT studies capture the transit of an administered radio-contrast material through vascular tissue. Generally, for contrast enhanced CT, a bolus of a radio-contrast material is intravenously administered to a patient, and a region of interest of the patient that includes the vascular tissue of interest is scanned. The radio-contrast material causes the x-ray density in the vascular tissue of interest to temporarily increase as the radio-contrast material flows through the vascular tissue, resulting in enhanced data. However, after administration of a contrast material, some patients experience idiosyncratic effects and certain patients may experience severe and potentially life-threatening allergic reactions. Contrast material may also induce kidney damage, and some patients have developed an acute deterioration of their kidney function. Generally, a larger contrast material volume results in higher contrast to noise (CNR) images, while a lower volume leads to lower CNR image. Unfortunately, as the contrast material volume increases, so does its associated risks.